The invention relates to processes for deacidizing natural gas by contacting with a solvent in two successive sections so as to eliminate acid compounds such as carbon dioxide (CO2) and/or hydrogen sulfide (H2S), then sulfur compounds such as mercaptans, carbon oxysulfide (COS) and/or carbon disulfide (CS2). The invention proposes a process for washing the hydrocarbons desorbed upon regeneration of the solvent.
Patent FR-B-2,600,554 describes a technology suited for elimination of the sulfur compounds that pollute a natural gas already containing CO2 and/or H2S. The principle consists in using two absorption sections. In the first absorption section, the CO2 and/or the H2S are collected by a totally regenerated solvent. The operating conditions in this section are determined so as to eliminate the major part of the CO2 and/or of the H2S. The gas thus obtained still contains sulfur compounds. It is then sent to a second absorption section wherein it is contacted with a solvent of the same nature as the solvent used in the first absorption section. This second section operates at a pressure that is similar to the pressure in the first absorption section, apart from the pressure drops, but at a lower temperature so as to favour absorption of the sulfur compounds.
Patent FR-B-2,600,554 proposes separate regeneration of the two solvent fractions coming from the two absorption sections. The solvent from the first absorption section is expanded so as to release the co-absorbed hydrocarbons and to eliminate a fraction of the acid gases absorbed. The solvent partly regenerated by expansion is then fed into a conventional regeneration column. The acid gases are desorbed and recovered at the top of the regeneration column. The perfectly regenerated solvent is then sent to a storage drum supplying both absorption sections. The solvent from the second absorption section is regenerated only by expansion, insofar as it essentially contains sulfur compounds. The solvent is advantageously heated prior to expansion. The regenerated solvent thus obtained is then sent to the storage drum supplying both absorption sections. The two gaseous fractions obtained upon expansion of the two solvent fractions from the two absorption sections are then mixed.
However, regeneration of the absorbent solution according to patent FR-B-2,600,554 involves drawbacks. In fact, upon expansion of the solvent coming from the first absorption section, an appreciable amount of acid compounds (CO2 and H2S) is released with the co-absorbed hydrocarbons. The gaseous effluent thus obtained is generally unusable because its acid compounds concentration exceeds the requirements imposed for a possible use as combustible gas, or possibly for marketing.
The aim of the present invention is to propose a treatment for freeing said gaseous effluent from acid compounds.
Since the CO2 and/or the H2S present in the gaseous effluent are to be eliminated, the conventional processes, an example of which is given in patent EP-B-021,479, propose washing by contacting the gaseous effluent with a perfectly regenerated solvent fraction. This conventional configuration consequently leads to an increase in the global solvent flow rate and to an increase in the consumption of the utilities required for regeneration of the absorbent solution.
Within the scope of a process using two absorption sections for treating a natural gas containing CO2 and/or H2S, as well as mercaptans, COS and/or CS2, the present invention aims to wash the gaseous hydrocarbons desorbed upon expansion of the solvent from the first absorption section with the solvent from the second absorption section.
The present invention relates to a process for deacidizing a natural gas containing acid compounds comprising at least compounds of the carbon dioxide and hydrogen sulfide group, and sulfur compounds comprising at least one of the compounds of the mercaptans, COS and CS2 group, comprising the following stages:
a) contacting said natural gas with a regenerated solvent in a primary absorption section so as to produce a gas poor in acid compounds and a solvent containing acid compounds and hydrocarbons,
b) contacting said gas poor in acid compounds with a regenerated solvent so as to produce a gas poor in acid compounds and in sulfur compounds and a solvent containing sulfur compounds,
c) expanding at least a first fraction of said solvent containing acid compounds and hydrocarbons so as to produce a gaseous effluent containing hydrocarbons and acid compounds, and a solvent containing acid compounds and freed of hydrocarbons,
d) contacting said gaseous effluent containing hydrocarbons and acid compounds with at least a first fraction of said solvent containing sulfur compounds so as to produce a gas containing hydrocarbons and sulfur compounds, and freed of acid compounds, as well as a solvent containing acid compounds.
The process according to the invention can also comprise at least one of stages e), f) or g):
e) distilling said solvent containing acid compounds and freed of hydrocarbons from stage c) and said solvent containing acid compounds from stage d) so as to obtain a gas containing acid compounds and a regenerated solvent that is recycled to at least one of stages a) and b),
f) distilling in a distillation column said solvent containing acid compounds and freed of hydrocarbons from stage c) and said solvent containing acid compounds from stage d) so as to obtain a gas containing acid compounds and, at an intermediate level between the top and the bottom of said column, a regenerated solvent that is recycled to at least stage b),
g) expanding said solvent containing acid compounds and freed of hydrocarbons from stage c) so as to produce a gas containing acid compounds and a solvent poor in acid compounds.
If the process according to the invention comprises stage g), it can also comprise stage h):
h) distilling in a distillation column at least a first fraction of said solvent poor in acid compounds from stage g) and said solvent containing acid compounds so as to produce a gas containing acid compounds and a regenerated solvent that is recycled to at least one of stages a) and b).
If the process according to the invention comprises stage h), it can also comprise stage i):
i) feeding said gas containing acid compounds from stage g) into said distillation column.
If the process according to the invention comprises stage i), it can also comprise stage j):
j) feeding a second fraction of said solvent poor in acid compounds from stage g) into said primary absorption section.
The process according to the invention can also comprise indiscriminately at least one of stages k), l), m) and n):
k) expanding a second fraction of said solvent containing sulfur compounds from stage d) so as to produce a gas and a regenerated solvent that is recycled to at least one of stages a) and b),
l) cooling a second fraction of said solvent containing acid compounds and hydrocarbons from stage a), then feeding it again into said primary absorption section,
m) prior to stage a), contacting at least a first fraction of said natural gas with a solution of methanol in water so as to produce a methanol-containing gaseous fraction, mixed with the remaining fraction of said natural gas, that is sent to stage a) with said natural gas,
n) after stage b), cooling said gas poor in acid compounds and sulfur compounds so as to produce at least a solution of methanol in water that is recycled to stage m).
The solvent used in the process according to the present invention can notably consist of an aqueous solution comprising methanol and a mixture of amine and alkanolamine representing each 0% to 60% by weight of the solvent, or of an aqueous solution comprising sulfolane and a mixture of amine and alkanolamine representing each 0% to 60% by weight of the solvent.
The present invention allows to wash the gaseous effluent obtained by expansion of the solvent from the first absorption section with a stream of solvent that has been previously used by the process to absorb sulfur compounds. Thus, washing of gaseous effluents does not require an increase in the global flow rate of solvent used by the process.